The Question:

Where did the saying "It ain't over until the fat lady sings"
come from?

The Answer:

Often mistakenly attributed to N.Y. Yankees Hall of Famer Yogi Berra—his
expression was "it ain't over till it's over"—this popular quote
originated in the following form:

The opera ain't over until the fat lady sings.

According to an article in the Washington
Post on June 3, 1978, San Antonio sports writer/broadcaster
Dan Cook first came up with that proverb about three years
before, in response to Ralph Carpenter's statement that "The rodeo
ain't over till the bull riders ride." According to Cecil
Adams, Cook says he first used it in a column around 1976, but
we haven't been able to confirm that.

What's certain is how the expression was popularized. Cook said
it on television in 1978, when the San Antonio Spurs were behind in a
series with the Washington Bullets. The Washington Bullets' manager,
Dick Motta, repeated it, and was widely quoted by the media. By the
time the Bullets came from behind to win the finals that year, the
saying was on thousands of T-shirts, "fat ladies" were coming to games
to cheer the team on, and the proverb's popularity was firmly
cemented.

(Motta, who had a solid grasp of grammar, was originally quoted
as saying "It's not over... It's like an opera. It doesn't end until
the fat lady starts singing and that hasn't happened in this one yet."
It immediately reverted to Cook's punchier version in
retellings.)

Finally, it should be noted that a Southern proverb, "church
ain't out until the fat lady sings," has been found in a 1976
publication. It's not certain which expression came first, and whether
one was influenced by the other.